Bond energies and formation enthalpies of mono- and polyradicals in nitroalkanes 1. Nitromethanes

The enthalpies of formation of nitromethane derivatives were obtained on the basis of experimental and literature data. The procedure for the calculation of the bond dissociation energies in nitromethanes from the atomization enthalpies and energies of nonvalent interactions of nitro groups was proposed. The calculated values were compared with the data on the thermal decomposition kinetics. The atomization enthalpies and energies of nonvalent interactions of nitro groups were also used for the calculation of the bond dissociation energies in radicals.     

Bond energies and the enthalpies of formation of mono- and polyradicals in nitroalkanes 2. Nitro derivatives of ethane and propane

Based on the experimental results and the published data, the enthalpies of formation of ethane and propane nitro derivatives were obtained for both the standard state and gas phase. The bond dissociation energies of the ethane and propane nitro derivatives were calculated using the enthalpies of atomization and the energies of nonvalent interactions of nitro groups. The calculated values were compared with the kinetic data on thermal decomposition. The bond dissociation energies in radicals of the ethane and propane nitro derivatives were also calculated using the enthalpies of atomization and the energies of nonvalent interactions of nitro groups. Regularities of changes in the bond dissociation energies of the ethane and propane nitro derivatives and their radicals were established.     

Extrapolation of finite cluster and crystal-orbital calculations on trans-polyacetylene

Sequences of total energies of trans-polyacetylene were obtained by either finite cluster or crystal-orbital calculations. Several different extrapolation methods were employed to improve the convergence of these two sequences. The properties of the extrapolation algorithms are discussed with emphasis on efficiency and reliability for short strings of physical input data. Four decimal digits were gained by extrapolation in the case of the finite cluster energies, but, at most, 2 decimal digits in the case of the crystal-orbital energies. The extrapolation of the finite cluster energies gave results that are almost as good as the results produced by the extrapolation of the crystal-orbital energies, and that are better than the unextrapolated crystal-orbital energies.     

Single-ion solvation free energies and the normal hydrogen electrode potential in methanol, acetonitrile, and dimethyl sulfoxide

The division of thermodynamic solvation free energies of electrolytes into contributions from individual ionic constituents is conventionally accomplished by using the single-ion solvation free energy of one reference ion, conventionally the proton, to set the single-ion scales. Thus, the determination of the free energy of solvation of the proton in various solvents is a fundamental issue of central importance in solution chemistry. In the present article, relative solvation free energies of ions and ion-solvent clusters in methanol, acetonitrile, and dimethyl sulfoxide (DMSO) have been determined using a combination of experimental and theoretical gas-phase free energies of formation, solution-phase reduction potentials and acid dissociation constants, and gas-phase clustering free energies. Applying the cluster pair approximation to differences between these relative solvation free energies leads to values of -263.5, -260.2, and -273.3 kcal/mol for the absolute solvation free energy of the proton in methanol, acetonitrile, and DMSO, respectively. The final absolute proton solvation free energies are used to assign absolute values for the normal hydrogen electrode potential and the solvation free energies of other single ions in the solvents mentioned above.     

SiH4与HX(X=F,Cl,Br,I)形成的二氢键复合物的结构特征及本质

本文就SiH4与HX形成的二氢键复合物的结构特征及本质进行了探讨。在MP2/6-311++G(3d,3p)水平优化、频率验证得到复合物的分子结构,通过分子间距离及电子密度等值线图,我们确认SiH4与卤化氢已形成了二氢键复合物。MP2/6-311++G(3d,3p)水平下进行BSSE校正后的结合能为2.703－4.439 KJ/mol。用对称匹配微绕理论(SAPT)对结合能进行分解,分解结果显示,SiH4匟X(X=F,Cl,Br,I)二氢键复合物中静电能对总吸引能的贡献小于28％,并且相对稳定,这就是说SiH4匟X二氢键复合物的本质并非静电作用,而是静电能、诱导能、色散能、交换能对总结合能的贡献都非常重要。     

Ab initio Calculation of Intermolecular Dispersion Energy and Induction Energy of Nitramide Dimer

The dispersion energies, induction energies and their exchange counterparts (exchange-dispersion and exchange-induction energies) between two interacting nitramide molecules at several separations are derived based upon symmetry-adapted perturbation theory (SAPT). The results show that (1) the effect of intramonomer electron correlation on dispersion energies and induction energies for nitramide dimer system is remarkable especially in the region near the van der Waals minimum distance (0.42 nm). (2) At smaller separations the dispersion energies and the induction energies are largely quenched by their exchange counterparts, and this case in induction interaction is much more remarkable than in dispersion interaction. (3) Since at shorter distances there exists the strong short-range interaction due to electron transfer which quickly decays and even disappears at larger separations, the two different R-dependency formulae of induction energies were found: one is ca. R^-12.7 at short distances, and the other ca. R^-7.0 at large separations. The latter R-dependency is similar to that (ca.R^-7.2) of dispersion. (4) In the case of strong polar interaction existing in nitramide dimer, the “true“ induction correlation terms of higher order than ^1Eind^(22) may be important.     

Molecular structures and energetics of the (TiO2)n (n = 1-4) clusters and their anions

The (TiO2)n clusters and their anions for n = 1-4 have been studied with coupled cluster theory [CCSD(T)] and density functional theory (DFT). For n > 1, numerous conformations are located for both the neutral and anionic clusters, and their relative energies are calculated at both the DFT and CCSD(T) levels. The CCSD(T) energies are extrapolated to the complete basis set limit for the monomer and dimer and calculated up to the triple-zeta level for the trimer and tetramer. The adiabatic and vertical electron detachment energies of the anionic clusters to the ground and first excited states of the neutral clusters are calculated at both levels and compared with the experimental results. The comparison allows for the definitive assignment of the ground-state structures of the anionic clusters. Anions of the dimer and tetramer are found to have very closely lying conformations within 2 kcal/mol at the CCSD(T) level, whereas that of the trimer does not. In addition, accurate clustering energies and heats of formation are calculated for the neutral clusters and compared with the available experimental data. Estimates of the titanium-oxygen bond energies show that they are stronger than the group VIB transition metal-oxygen bonds except for tungsten. The atomization energies of these clusters display much stronger basis set dependence than the clustering energies. This allows the calculation of more accurate heats of formation for larger clusters on the basis of calculated clustering energies.     

Strain Energies in Homoatomic Nitrogen Clusters N(4), N(6), and N(8)

Strain energies and resonance energies can be obtained as the energy changes for appropriate homodesmotic reactions using ab initio calculated total energies as the energies of the reactants and products involved. Homodesmotic reactions conserve bond types and preserve valence environments at all atoms, requirements that favor the cancellation of basis set and electron correlation errors in the ab initio energies. In this paper we calculate strain energies and resonance energies for N(4), N(6), and N(8) clusters in a number of chemically significant but, for nitrogen, hypothetical structural forms. The nitrogen cluster strain energies are generally of the same order of magnitude as those of isostructural hydrocarbon clusters, and individual differences can be explained by using the ring strain additivity rule and recognizing the effect of the presence of lone pairs of electrons on nitrogen clusters but not on the hydrocarbons. Resonance energies of the nitrogen clusters are much smaller than those of the comparable aromatic hydrocarbons. The differences can be rationalized by considering the relative strengths of CC and NN single and double bonds. Strain and resonance energies of nitrogen clusters are compared with those previously reported for homoatomic clusters of phosphorus and arsenic. Trends through the series are remarkably similar, but strain energies for clusters from lower periods are progressively smaller. Strain and resonance have been important organizing concepts in organic chemistry for many years. Estimates of corresponding parameters for inorganic analogs are only now becoming available.     

Pi* orbital system of alternating phenyl and ethynyl groups: measurements and calculations

The temporary anion states of a series of alternating phenyl-ethynyl compounds are studied by means of electron transmission spectroscopy. Calculations of the virtual orbital energies of these compounds are computed with ab initio HF methods as well as DFT, and excellent correlations with the experimental vertical attachment energies are obtained. Scaled orbital energies for long-chain molecules are used to predict the vertical attachment energies of these compounds. In the absence of scaling, HOMO-LUMO gaps computed by DFT are found to be in substantial disagreement with gas-phase data. Such discrepancies may cause significant errors in theoretical studies of molecular conductance.     

The enthalpies of formation and reorganization of aromatic radicals

The enthalpies of formation of some biphenyl derivatives were determined. A "double difference" method for calculating the enthalpies of formation of aromatic radicals and the bond dissociation energies was proposed. The enthalpies of formation of the radicals biphenyl, diphenyl oxide, and phenyl oxide were determined. The energies of reorganization of these radicals as well as phenyl and 4-, 3-, and 2-pyridyls were calculated. The sums of the energies of the chemical bonds in the molecular moieties transformed into radicals upon the decomposition of chemical compounds were found to be constant for different compounds. The energies of the chemical bonds in arenes were determined.     

Experimental investigation of double ionization to ground and electronically excited states of chlorobenzene, 1,2-, 1,3- and 1,4-dichlorobenzene, 1,2,3-, 1,2,4- and 1,3,5-trichlorobenzene, 1,2,3,4-and 1,2,4,5-tetrachlorobenzene and pentachlorobenzene dications

Double-charge-transfer spectrometry was used to measure double-ionization energies to ground and electronically excited states of various chlorobenzenes. Since OH⁺ was the projectile ion used in these experiments, it is probable that triplet states of the dications were populated because of spin conservation in the double-electron-capture reactions which are the basis of this type of spectrometry. The lowest double-ionization energies for all the molecules studied are within ±0.3 eV of 25.9 eV, except that for 1,3-dichlorobenzene, which is at 26.6 eV. In general, double-ionization energies to three higher lying states (or groups of states) were measured for each molecule. The energies of these states are the same, within experimental uncertainties, for the three trichlorobenzenes, two tetrachlorobenzenes and pentachlorobenzene dications, suggesting that they have the same or very similar distributions of triplet-state energies.     

Highly Accurate CCSD(T) and DFT–SAPT Stabilization Energies of H‐Bonded and Stacked Structures of the Uracil Dimer

The CCSD(T) interaction energies for the H‐bonded and stacked structures of the uracil dimer are determined at the aug‐cc‐pVDZ and aug‐cc‐pVTZ levels. On the basis of these calculations we can construct the CCSD(T) interaction energies at the complete basis set (CBS) limit. The most accurate energies, based either on direct extrapolation of the CCSD(T) correlation energies obtained with the aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets or on the sum of extrapolated MP2 interaction energies (from aug‐cc‐pVTZ and aug‐cc‐pVQZ basis sets) and extrapolated ΔCCSD(T) correction terms [difference between CCSD(T) and MP2 interaction energies] differ only slightly, which demonstrates the reliability and robustness of both techniques. The latter values, which represent new standards for the H‐bonding and stacking structures of the uracil dimer, differ from the previously published data for the S22 set by a small amount. This suggests that interaction energies of the S22 set are generated with chemical accuracy. The most accurate CCSD(T)/CBS interaction energies are compared with interaction energies obtained from various computational procedures, namely the SCS–MP2 (SCS: spin‐component‐scaled), SCS(MI)–MP2 (MI: molecular interaction), MP3, dispersion‐augmented DFT (DFT–D), M06–2X, and DFT–SAPT (SAPT: symmetry‐adapted perturbation theory) methods. Among these techniques, the best results are obtained with the SCS(MI)–MP2 method. Remarkably good binding energies are also obtained with the DFT–SAPT method. Both DFT techniques tested yield similarly good interaction energies. The large magnitude of the stacking energy for the uracil dimer, compared to that of the benzene dimer, is explained by attractive electrostatic interactions present in the stacked uracil dimer. These interactions force both subsystems to approach each other and the dispersion energy benefits from a shorter intersystem separation.     

The effect of atomic and extra-atomic relaxation on atomic binding energies

An equivalent-cores-relaxation model is given for calculating atomic binding energies from orbital energies using only ground-state atomic properties. The agreement with experiment is excellent for the noble gases. On the basis of present knowledge of atomic relaxation, the phenomenon of “extra-atomic relaxation”, in which electronic charge is attracted toward a hole-state atom, is shown to have an important effect in lowering atomic core-level binding energies in condensed phases. This will affect the interpretation of most core-level binding energies measured to date.     

Electron binding energies of aqueous alkali and halide ions: EUV photoelectron spectroscopy of liquid solutions and combined ab initio and molecular dynamics calculations

Photoelectron spectroscopy combined with the liquid microjet technique enables the direct probing of the electronic structure of aqueous solutions. We report measured and calculated lowest vertical electron binding energies of aqueous alkali cations and halide anions. In some cases, ejection from deeper electronic levels of the solute could be observed. Electron binding energies of a given aqueous ion are found to be independent of the counterion and the salt concentration. The experimental results are complemented by ab initio calculations, at the MP2 and CCSD(T) level, of the ionization energies of these prototype ions in the aqueous phase. The solvent effect was accounted for in the electronic structure calculations in two ways. An explicit inclusion of discrete water molecules using a set of snapshots from an equilibrium classical molecular dynamics simulations and a fractional charge representation of solvent molecules give good results for halide ions. The electron binding energies of alkali cations computed with this approach tend to be overestimated. On the other hand, the polarizable continuum model, which strictly provides adiabatic binding energies, performs well for the alkali cations but fails for the halides. Photon energies in the experiment were in the EUV region (typically 100 eV) for which the technique is probing the top layers of the liquid sample. Hence, the reported energies of aqueous ions are closely connected with both structures and chemical reactivity at the liquid interface, for example, in atmospheric aerosol particles, as well as fundamental bulk solvation properties.     

Bond energies in chloroethanes and chloroethyl radicals

C? Cl and C? C bond energies in the chloroethanes and C? H, C? Cl, and C? C bond energies in the chloroethyl radicals are calculated from known heats of formation of chloroethanes and chloroethylenes and known C? H bond energies in chloroethanes. The results obtained show a dependence of bond energy on the isomeric structure of the molecules and radicals and on the type of bond broken (primary, secondary, or tertiary). Heats of formation and bond energies estimated from group property additivity rules are in close agreement with experimental values.     

Threshold dissociation and molecular modeling of transition metal complexes of flavonoids

The relative threshold dissociation energies of a series of flavonoid/transition metal/auxiliary ligand complexes of the type [MII (flavonoid - H) auxiliary ligand]+ formed by electrospray ionization (ESI) were measured by energy-variable collisionally activated dissociation (CAD) in a quadrupole ion trap (QIT). For each of the isomeric flavonoid diglycoside pairs, the rutinoside (with a 1-6 inter-saccharide linkage) requires a greater CAD energy and thus has a higher dissociation threshold than its neohesperidoside (with a 1-2 inter-saccharide linkage) isomer. Likewise, the threshold energies of complexes containing flavones are higher than those containing flavanones. The monoglycoside isomers also have characteristic threshold energies. The flavonoids that are glycosylated at the 3-O- position tend to have lower threshold energies than those glycosylated at the 7-O- or 4'-O- position, and those that are C- bonded have lower threshold energies than the O- bonded isomers. The structural features that substantially influence the threshold energies include the aglycon type (flavanone versus flavone), the type of disaccharide (rutinose versus neohesperidose), and the linkage type (O- bonded versus C- bonded). Various computational means were applied to probe the structures and conformations of the complexes and to rationalize the differences in threshold energies of isomeric flavonoids. The most favorable coordination geometry of the complexes has a plane-angle of about 62 degrees , which means that the deprotonated flavonoid and 2,2'-bipyridine within a complex do not reside on the same plane. Stable conformations of five cobalt complexes and five deprotonated flavonoids were identified. The conformations were combined with the point charges and helium accessible surface areas to explain qualitatively the differences in threshold energies for isomeric flavonoids.     

Simulations of solvation free energies and solubilities in supercritical solvents

Computer simulations are used to study solvation free energies and solubilities in supercritical solvents. Solvation free energies are calculated using the particle insertion method. The equilibrium solvent configurations required for these calculations are based on molecular dynamics simulations employing model solvent potentials previously tuned to reproduce liquid-vapor coexistence properties of the fluids Xe, C(2)H(6), CO(2), and CHF(3). Solutes are represented by all-atom potentials based on ab initio calculations and the OPLS-AA parameter set. Without any tuning of the intermolecular potentials, such calculations are found to reproduce the solvation free energies of a variety of typical solid solutes with an average accuracy of +/-2 kJmol. Further calculations on simple model solutes are also used to explore general aspects of solvation free energies in supercritical solvents. Comparisons of solutes in Lennard-Jones and hard-sphere representations of Xe show that solvation free energies and thus solubilities are not significantly influenced by solvent density fluctuations near the critical point. The solvation enthalpy and entropy do couple to these fluctuations and diverge similarly to solute partial molar volumes. Solvation free energies are also found to be little affected by the local density augmentation characteristic of the compressible regime. In contrast to solute-solvent interaction energies, which often provide a direct measure of local solvent densities, solvation free energies are remarkably insensitive to the presence of local density augmentation.     

5-氟胞嘧啶气相及水助质子转移异构化的理论研究

采用密度泛函B3LYP/6-311G**方法,对6种5-氟胞嘧啶异构体孤立分子的稳定性及质子转移引起的酮式-烯醇式、氨基式-亚胺式互变异构反应机理进行了计算研究,获得了零点能、吉布斯自由能及质子转移过程的反应焓、活化能、活化吉布斯自由能和速率常数等参数.计算结果表明,气相中烯醇-氨基式FC4是最稳定的异构体.分子内质子转移设计了FC1→FC2和FC1→FC6两条通道,分别标记为P(1)和P(2),各通道速控步骤的活化能和速率常数分别为155.9 kJ·mol-1,4.70×10-15 s-1和173.1 kJ·mol-1,1.41×10-18 s-1.水助催化时,相应通道P(3) 和P(4) 速控步骤的活化能和速率常数分别为51.0 kJ·mol-1,1.41×103 s-1和88.2 kJ·mol-1,4.53×10-3 s-1.可见,水分子的加入极大地降低了质子转移的活化能垒.另外发现,水分子参与形成协同的双质子转移机理比水助单质子转移机理更利于降低活化能垒.     

Theoretical study on the isomerization and tautomerism in barbituric acid

Isomerization and tautomerism of 16 isomers of barbituric acid (BA) were studied at the MP2 and B3LYP levels of theory. Activation energies (E _a), imaginary frequencies (υ), and Gibbs free energies (ΔG ^#) of the amine-imine and keto-enol tautomerisms and O–H internal rotations were calculated. The activation energies of amine-imine tautomerisms were in the range of 110–200 kJ/mol and for keto-enol tautomerisms were larger than 200 kJ/mol. The calculated activation energies of internal O–H rotations were smaller than 60 kJ/mol. Effect of micro-hydration on the transition state structures and activation energies of the tautomerisms were also investigated. Water molecule catalyzed the tautomerisms and decreased the activation energies of both the amine-imine and keto-enol tautomerisms about 100–120 kJ/mol.